1. Field of the Invention
This invention relates to the field of linear voltage regulators, and particularly to short-circuit protection circuits and methods for such regulators.
2. Description of the Related Art
A linear voltage regulator includes an error amplifier and a xe2x80x9cpassxe2x80x9d transistor. The error amplifier inputs a setpoint voltage and a feedback signal representing the regulator""s output voltage, and outputs a drive signal which varies with the error between its inputs. The pass transistor""s gate (assuming the pass transistor is a field-effect transistor (FET)) is connected to receive the drive signal, and its drain-source circuit is connected between an input voltage and the regulator""s output voltage terminal. The error amplifier""s drive signal causes the pass transistor to conduct a current to the output voltage terminal which maintains the output voltage at a desired level.
One problem which may afflict a linear voltage regulator is a short-circuit between the output voltage terminal and ground. When this occurs, the output voltage droops, and the error amplifier responds by increasing the drive signal. This results in the pass transistor carrying a high current, which may damage or destroy the transistor.
To prevent damage to the pass transistor, many linear regulators employ some form of short-circuit protection. Many protection methods require the use of an external sensing element such as a resistor to detect a short. However, this approach increases the regulator""s cost, complexity, and power dissipation, and requires an extra pin on the integrated circuit (IC) containing the regulator circuitry.
One method which avoids the need for an external resistor is referred to as xe2x80x9chiccup modexe2x80x9d short-circuit protection, which operates as follows: once a short-circuit condition is detected, closed-loop operation of the regulator is suspended. The pass transistor is turned on briefly, and the output voltage measured. If the output voltage is below a predetermined threshold voltage, indicating that the short-circuit persists, the pass transistor is turned off. After a longer period the pass transistor is turned on again, and the output voltage again measured and compared with the threshold voltage. This pattern is repeated indefinitely, unless the short-circuit clears and the output voltage recovers.
The ratio of the pass transistor""s on-time to off-time while the regulator is in hiccup mode is typically controlled by some sort of timing circuit. One approach is illustrated in the ADM1050 voltage regulator controller from Analog Devices, Inc., which employs an internal digital timer and oscillator to handle hiccup mode timing. However, this approach can be silicon intensive, particularly when the integrated circuit (IC) is fabricated using analog processes which are not optimized for digital circuitry. Another implementation is used in the UCC3837 linear regulator controller from Unitrode Corp., which requires the use of an external timing capacitor to provide the necessary timing. However, the external capacitor introduces an extra cost, as well as requiring a dedicated IC package pin.
A hiccup-mode short circuit protection circuit and method suitable for use with a linear voltage regulator which employs a FET pass transistor is presented. The invention avoids the problems discussed above, requiring neither an internal timer nor an extra pin for a timing capacitor.
The invention uses the capacitance of the FET pass transistor""s gate as a hiccup-mode timing element. The regulator""s output voltage is monitored, and when it droops below a first predetermined threshold indicative of a short-circuit condition, the amplifier""s drive signal is essentially disconnected from the pass transistor. While the short-circuit condition persists, a first current is provided to charge the pass transistor""s gate capacitance. When the gate voltage rises above a second predetermined threshold, a second current is provided to further charge the gate capacitance. When the gate voltage rises above a third predetermined threshold, the gate capacitance is discharged. The gate capacitance is cyclically charged and discharged in this way indefinitely, unless the output voltage rises enough to indicate that the short-circuit condition has clearedxe2x80x94in which case the amplifier""s drive signal is reconnected to the pass transistor""s gate and normal operation resumed.
In a preferred embodiment, a first current source charges the gate capacitance slowly until the pass transistor""s turn-on voltage is reached, at which point a second, larger current source is connected to quickly ramp up the gate voltage. To reduce average power consumption, the magnitudes of the first and second currents and the values of the second and third predetermined threshold voltages are chosen such that the ON duty cycle for the pass transistor is about 10%.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.